ERDC/CHL CHETN-I-65
June 2002
heights comparable to the incident wave height, or slightly lower. The prototype gauge shows
wave heights 10 to 20 percent higher than incident. The discrepancy may be due to several
factors. The model gauge was located in the center of the dredged entrance, whereas, the
prototype gauge was located shoreward of the dredged area, in shallower water, since it could
not be placed in the navigation channel. Waves shoaling over the dredged slope would be
expected to increase in height. In addition, bathymetry in the prototype changes with time and
the entrance area was somewhat shallower than the ideal project depths molded into the physical
model. Tidal currents probably also influenced the prototype data. The prototype gauge is in line
with ebb current jets flowing out of the entrance gap. Interactions between ebb currents and
incoming waves would tend to increase wave height in the prototype.
At the inner harbor gauge location, physical model wave heights tend to be comparable to or
higher than the prototype data. Overall, the physical model effectively predicted decay of wave
height between incidence and this sheltered location. Differences in wave gauge locations may
contribute to model/prototype differences. Physical model wave gauges were located in the
center line of the channel, whereas, the prototype inner harbor gauge, for practical reasons, was
placed along the channel flank, in a more protected location. Inner gauge site 1 was used for
these comparisons.
COMPARISON OF PROTOTYPE AND NUMERICAL MODEL WAVE ESTIMATES:
Numerical model results are also shown in the comparison plots (Figures 8-11). The original
HARBD model results are for the wave period and direction at the model boundary best
matching physical model wave parameters. Alternative 6 in the original HARBD study was used
as a best match to the project condition. HARBD was run only for regular (monochromatic)
waves. HARBD results show a diminishing wave height as waves progress from incidence into
the sheltered part of the channel. HARBD results are close to nondirectional buoy results, but
considerably higher than the inner harbor gauge results. Wave height amplification factors are
greater for HARBD than for the physical model at all but the most inner end of the channel.
HARBD, as applied in the original study, suffered several major limitations, including regular
(monochromatic) waves, no wave breaking, and restricted grid size and coverage area. The
regular wave representation can lead to strong reflection patterns, wave heights significantly
greater than incident wave height outside the harbor entrance, and erratic wave height variations
over short distances. The lack of wave breaking in HARBD is also a serious limitation for the
Morro Bay Harbor application that may lead to overprediction of wave heights.
The most current technology for numerical harbor wave modeling, CGWAVE (Demirbilek and
Panchang 1998), was activated and run for four comparison cases as part of this monitoring
study. CGWAVE results are also shown in Figures 8-11. CGWAVE runs were designed to
match physical model experiments, including unidirectional, spectral waves, similar bathymetry,
and wave breaking. The CGWAVE model domain extended significantly further seaward than
the HARBD domain in previous studies. CGWAVE results compare much more favorably than
HARBD results with physical model data. This is partly attributable to CGWAVE being a more
comprehensive model and partly to CGWAVE being expressly configured to match physical
model conditions. CGWAVE matches the inner harbor prototype gauge well. As with the
physical model, it falls below the nondirectional buoy data, helping to support the explanation
that gauge placement, shoaling, and currents may be affecting the prototype data at this location.
11